1. Technical Field
The present invention relates to a polyimide precursor composition containing a polyamic acid alkyl ester, and a polyimide film having a high imidization degree obtainable from the polyimide precursor composition.
2. Background Art
A polyimide is a polymer material excellent in the heat resistance, the mechanical strength, the electrical properties and the solvent resistance and is widely used e.g. in an electronic material field as a polyimide film such as an insulating film, a protective film or a liquid crystal alignment film. To industrially obtain such a polyimide film, a common method is to prepare a coating liquid having a polyimide or a polyimide precursor dissolved in a solvent, apply and bake it.
As the polyimide precursor, a polyamic acid, a polyamic acid ester, etc. are used. They are advantageous in that they are highly soluble in a solvent as compared with a polyimide, and accordingly, in the case of the above method of using a coating liquid to obtain a polyimide film, the structure of the polyimide, the type of the solvent to be used, etc. can relatively freely be selected. The coating film of such a polyimide precursor is imidized by being baked at from 200 to 400° C. to form a polyimide film.
A polyamic acid is easily obtained by reacting a diamine with a tetracarboxylic dianhydride, and as this reaction is a reversible reaction, a reverse reaction to the diamine and the acid dianhydride also proceeds at the same time as the imidization by the heat at the time of baking. As a result, the molecular weight of the polyimide to be obtained is lower than the starting polyamic acid, and such may impair the properties of the polyimide film. On the other hand, with the polyamic acid ester, no reverse reaction such as that of the polyamic acid occurs, and thus there will be no decrease in the molecular weight at the time of baking, however, imidization is less likely to proceed by heat as compared with the polyamic acid, and imidization at a temperature higher than that for the polyamic acid will be required.
In general, the higher the above baking temperature, the higher the imidization degree from the polyimide precursor to the polyimide. Although it is not necessarily required to achieve an imidization degree of a polyimide film of 100% depending upon the purpose of use, achievement of a desired imidization degree at a lower temperature is advantageous in view of the energy cost and is advantageous in that a polyimide film can be formed also on a substrate having low heat resistance.
To solve such problems, heretofore, a method of mixing various compounds having an imidization accelerating effect with a polyimide precursor composition at the time of imidization by heating has been proposed. For example, as a compound which can imidize a polyamic acid by baking at a low temperature, an amino acid compound has been disclosed (Patent Document 1). Further, as a compound to lower the imidization temperature of a polyamic acid alkyl ester to the vicinity of 150° C., an amine compound such as phenetylamine or dodecylamine has been disclosed (Non-Patent Document 1).
Further, it is disclosed that a thermal base generator which is a neutral compound which is decomposed by heat to generate a secondary amine does not form a salt with the carboxy group in the polyamic acid when not heated, and accordingly the polyimide precursor composition has good storage stability, and accordingly the thermal base generator is useful as a thermal imidization accelerator for a polyamic acid (Patent Document 2). It is also disclosed that as this thermal base generator can be used also as a thermal imidization accelerator for a polyamic acid ester, it can be used regardless of the type of the polyimide precursor.
From the viewpoint different from the imidization degree, when a polyimide film is to be formed on a substrate made of an inorganic material, insufficiency in the adhesion to the substrate is problematic in some cases. In such a case, a method of using an organic silicon compound called a silane coupling agent is commonly employed. As a method of using the silane coupling agent, a method of coating the surface of a substrate with a silane coupling agent, followed by forming a polyimide film, a method of adding a silane coupling agent to a coating liquid for preparation of a polyimide film, a method of reacting a silane coupling agent with e.g. the terminal of the polyimide or the polyimide precursor, and the like have been known.